I'll have a crack at that and say that for me, it's Streamlight. They've always made the lights that better/best suited my needs, with the features I want and need, at a reasonable price I can afford, and have a track record of decades of flawless service with me, including a lot of professional daily use in fairly demanding real world conditions. It's tough for me to beat that. I hear they have first rate service and support too, although I've never used it. They also document their product's performance like few others do, so you know what you're getting. That separates them from many other manufacturers.
I imagine lots of people will also tell you Surefires best suit their particular needs, and they certainly also have a great and long reputation. I just have no personal experience with them, so others can speak to that.
Some of the choice comes down to whose designs better suit your needs and preferences. Streamlight likely covers more user segments with a much broader product line, although their target market segments do have some overlap.
Specifically, I'd recommend you first look at the Streamlight product line, find a couple of models that you might be interested in, then look at the similar product(s) from Surefire and compare them that way. I think that would get you to an answer fairly easily based on your own needs, rather than on mine or some others' needs. More info on your intended usages and features / capabilities you want / need will help people here help you narrow things down a bit.
At least you're starting out by looking at a couple of good manufacturers!